A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In the conventional lithographic projection apparatus, during photolithographic processes, an article, such as a wafer or reticle, is clamped on an article support by a clamping force that may range from vacuum pressure forces, electrostatic forces, intermolecular binding forces or just the gravitational force. The article support defines a plane in the form of a plurality of protrusions defining an even flat surface on which the wafer or reticle is held. Tiny variations in the height of these protrusions are detrimental to image resolution, since a small deflection of the article from an ideal plane orientation may result in rotation of the wafer and a resulting overlay error due to this rotation. In addition, such height variations of the article support may result in height variation of the article that is supported thereby. During the lithographic process, such height variations may affect image resolution due to a limited focal distance of the projection system. Therefore, it is desirable to have an ideal flat article support.
European patent application EP0947884 describes a lithographic apparatus having a substrate holder in which protrusions are arranged to improve the flatness of the substrate. These protrusions have a general diameter of 0.5 mm and are located generally at a distance of 3 mm away from each other, thereby forming a bed of supporting members that support the substrate. Due to the relative large spaces in between the protrusions, any contamination that is present generally does not form an obstruction for the flatness of the substrate, since the contamination will be lying in between the protrusions and will not lift the substrate locally.
In the context of this application, the “article” may be any of the above-mentioned terms wafer, reticle, mask, or substrate, more specifically terms such as a substrate to be processed in manufacturing devices employing lithographic projection techniques; or a lithographic projection mask or mask blank in a lithographic projection apparatus, a mask handling apparatus such as mask inspection or cleaning apparatus, or a mask manufacturing apparatus or any other article or optical element that is clamped in the light path of the radiation system.
It has come to the attention of the inventors that this clamping force may cause problems when the article is released from the article support.
A common way to release the article from the article support is by stepping up a release force exerted by an ejection pin mechanism (e-pins), which engages at the article and provides a releasing force to disengage the article from the article support. The energy that is built up in the article during this stepping up is converted into displacement by subsequent release of the article surface from the article support surface. However, this built up energy may also cause damage to the article or article support. In general, by releasing the article from the article support, an uneven load distribution of the protrusions is obtained. In particular, the protrusions near a boundary are very much unevenly loaded and by slight rotations of the article and slip between the article and the supporting protrusions, the protrusions may wear and the article support will eventually loose its perfectly level plane of support due to tiny variations of the height and form of protrusions that are introduced during this release action.